Loess deposits in the low latitudes of East Asia reveal the ~20-kyr precipitation cycle

The cycle of precipitation change is key to understanding the driving mechanism of the East Asian summer monsoon (EASM). However, the dominant cycles of EASM precipitation revealed by different proxy indicators are inconsistent, leading to the “Chinese 100 kyr problem”. In this study, we examine a high-resolution, approximately 350,000-year record from a low-latitude loess profile in China. Our analyses show that variations in the ratio of dithionite−citrate−bicarbonate extractable iron to total iron are dominated by the ~20-kyr cycle, reflecting changes in precipitation. In contrast, magnetic susceptibility varies with the ~100-kyr cycle and may be mainly controlled by temperature-induced redox processes or precipitation-induced signal smoothing. Our results suggest that changes in the EASM, as indicated by precipitation in this region, are mainly forced by precession-dominated insolation variations, and that precipitation and temperature may have varied with different cycles over the past ~350,000 years.


Profile and sampling
The eolian sediments in the Madang profile are mainly characterised by their brownishyellow colour, massive structure and vertical joints developed on outcrops, which are typical characteristics of loess.According to the sedimentary characteristics, the strata of the Madang profile can be divided into eight layers from top to bottom, which roughly correspond to the stratigraphic units L1, S1, L2, S2, L3, S3 and L4 of the CLP (Supplementary Fig. 3).The loess units are generally brownish yellow and the paleosol units are yellowish brown, forming a typical loess-paleosol sequence (Supplementary Table 1).The sampling interval is 5 cm for the upper part (<13.2 m) and 10 cm for the lower part (>13.2 m), with each sample being approximately 2 cm thick.A total of 291 bulk samples were collected for measurements of grain size, magnetic susceptibility (MS), dithionite-citrate-bicarbonate (DCB) extractable iron (FeD), redness and hematite content.In addition, 6 samples were collected for optical stimulated luminescence (OSL) dating.

Grain size analysis
2 g of loess sample is placed in a beaker, and then 10 ml of 10% H2O2 is added and heated.Next, 10 ml of 10% HCl is added and the mixture is boiled.The beaker is then filled with distilled water and left to settle for 12 hours before the supernatant is decanted.Then 10 ml of 0.05 ml/L (NaPO3)6 dispersant is added, and the sample is shaken with an ultrasonic cleaner for 15 minutes.Finally, the sample is measured using a Malvern Panalytical Mastersizer2000 laser particle sizer.These measurements were carried out at the Laboratory of Surface Processes and Environment Evolution, Nanjing University.

Redness measurement
The redness of the sample was measured using a Konica-Minolta CM-700D spectrophotometer, which has a wavelength range of 400-700 nm and a test interval of 10 nm.The following steps were followed during the testing process: (1) The sample was naturally dried and ground to a particle size of less than 200 mesh.(2) The spectrophotometer was calibrated using a standard calibration white plate (CM-A177) with a standard deviation of spectral reflectance of less than 0.1% and a standard deviation of chromaticity (ΔE*ab) less than 0.04.(3) A suitable amount of sample was placed on a glass disc and flattened.(4) Three random measurements of the sample surface were taken with spectrophotometer to obtain the redness (a*) value of the sample.The redness measurements were conducted at the Institute of Geology, Chinese Academy of Geological Sciences.

Hematite measurement
The Hematite content is measured by diffuse reflectance spectroscopy (DRS)  Color.
The samples are first ground to less than 200 mesh using an agate mortar, placed on glass slides, diluted with distilled water to a slurry and spread evenly, dried at low temperature (< 40 °C), and spectroscopically scanned using a Lambd 900 UV/VIS/NIR spectrophotometer from Perkin-Elmer, with a measurement interval of 2 nm, the measured band is 400-700 nm, and the visible band is divided into six different color bands, violet = 400-450 nm, blue = 450-490 nm, green = 490-560 nm, yellow = 560-590 nm, orange = 590-630 nm, red = 630-700 nm.
Stepwise multiple linear regression analysis is performed on the added hematite content and reflectance of each colour band (Supplementary

Grain size distribution
The representative particle size frequency distribution curves from each unit of the Madang profile are relatively consistent, and all show a morphologically asymmetric distribution, with silt as the major component, and none of them show a coarse tail.
The particle size frequency distribution curves of the Madang samples show that there exist three peaks with corresponding mode sizes of ~0.9 μm, ~7 μm and ~30 μm respectively.Each peak represents a component corresponding to a different dynamic process.The <1 μm particles may be the pedogenic component 3 , the 1-10 μm particles are the distal component, and the 16-32 μm particles are the proximal component (Supplementary Fig. 4).Therefore, the Madang loess has typical characteristics of dust sediments, consistent with the typical loess of the Loess Plateau.

. 3 .
Outcrops of the Madang profile and lithology of the eolian sediments.(a)Stratigraphic subdivision following the scheme used in the Loess Plateau.The white dashed line represents the stratigraphic boundary.The visible strata thickness is 13.2 m; (b-d) The closeup photos of unit L1, S1 and L2, respectively.

Supplementary Fig. 4 .
Grain size frequency curve of samples from different units of the Madang profile.

Supplementary Table 1. Stratigraphic subdivision and lithological description of the Madang profile.
2 . Before conducting the DRS tests, the samples are pre-treated by the DCB dissolution method,

Table 5 . Tie points between the depth of the Madang profile and the age of the Luochuan profile
4.